Probably the most common, widely used object and people counting system consists simply of an illuminating device, such as a light bulb, and a light sensitive receiver, such as a photocell. The photocell and light bulb are located at opposite sides of an entrance to the area so that as people and objects enter or leave the area they interrupt the light path between the bulb and cell, thereby modulating the cell's output. By counting the number of such interruptions absolute traffic flow is ascertained. Obviously, there are other more sophisticated variations of this perimeter monitoring system, nevertheless, all suffer from the same limitations: they cannot distinguish movement in and out of the area or if more than one person is passing through the perimeter at one time; the reason being that they provide only a zero dimensioned view of the perimeter.
It is possible to provide a perimeter monitoring system that can distinguish in and out movement. Such a system may use adjacent bulbs and cells so that as people or objects pass through, two interruptions occur in a sequence that reflects the direction of movement. Nevertheless, this system cannot distinguish side-by-side movement of objects, and, although it provides a means for ascertaining traffic flow in and out of the area monitored, it cannot be used to detect the number of people and objects in the area at any instant of time because it cannot distinguish side-by-side movement. Hence, it is simply a traffic counter.
The evolution of these systems includes the use of a TV camera located above the monitored area in order to count the number of people and objects in the area, at any instant, by counting the light or dark spots that objects standing in the area produce in the TV picture.
Indeed, such a system does not suffer from the "one dimensional" limitations of perimeter monitoring systems, nevertheless, it does present serious disadvantages; principal among these are high cost and inaccuracy. A factor contributing to the expense is that the entire TV picture must be monitored to detect the presence of objects. The inaccuracy arises from the object masking that occurs at the perimeter of the areas. People and objects substantially below the camera are seen from above and thus the area they are seen to occupy on the floor is correct. However, looking towards the perimeter the height of objects and width of objects are seen and, thus, the observed area tends to increase beyond what it actually is. Likewise, the fact that the camera is taking a "perspective" view (rather than "plan") means that an object located next to another object, but further from the camera, is masked and will not be seen. Where the floor to camera distance is small, a wide angle lens is often used to see the entire area, but that aggravates these viewing problems. Consequently, these overhead camera systems are expensive, inaccurate and often impractical, especially if used for monitoring small, wide areas from a low height, directly above the floor.